Reduction of distortion in signal transmission systems



Dec. y12, -1939. 1 R, DAVEY ET AL 2,182,841

REDUCTION oF DISTORTION 1N SIGNAL TRANSMISSION SYSTEMS Filed May 19, 1958 T0 REC.

LOOP

A TTORNE V Patented Dec. 12, 1939 lUNITED STATES A 2,182,841 PATENT OFFICE RDUCTION F DISTORTION IN SIGNAL TRANSDIISSON SYSTEMS James R. Davey, Kew Gardens, N. Y., and John L. Hysko, Ridgewood, N. J., assignors to Bell Telephone Laboratories, Incorporated,

New

York, N. Y., a corporation of New York Application May 19, 1938, Serial No. 208,790

11 Claims.

v system,

Another object is to reduce signal distortion in a telegraph transmission system, including a type of distortion known as telegraph bias.

It has been found in telegraphy that marking and spacing signals in transmission over a system are subject to distortion such that at the receiving end of thesystem the durations of the marking and spacing signals are different from those they had at the transmitting end of the system, and will vary from time to time. This type of distortion, commonly called telegraph bias, may be due to chang'es in repeating relay adjustments, battery voltages or received telegraph currents caused by changes in line equivalent. Heretofore,

this telegraph bias has been reduced by utilizing the received signal variations to control the bias on the control grid of a vacuum tube detector in the telegraph receiving circuit, to vary the gain of that circuit so as to compensate for the received level variations and thus prevent them from aiecting the operation of the receiving relay.

In accordance with the present invention, the above objects are attained in an improved design of voice-frequency carrier telegraph receiving circuit, the operation of which is practically independent of changes in vacuum tubes, receiving relays and battery voltages, and in which the signal distortion is negligible even at the lowest carrier frequencies.

A feature of this circuit is an easily adjustable level compensator circuit which will maintain substantially unbiased signal reception over a Wide range of amplitude levels of the received current, and which is practically free of any drift effects, that is, a circuit in which the compensating voltage is a function of the incoming level only, and not of the signal characters being received. The circuit employs a suppressor grid of a multigrid vacuum tube controlling the operation of the receiving telegraph relay, to obtain a readily adjustable current for controlling the gain of one or more amplifying tubes in the circuit to obtain the 'desired compensation.

An auxiliary feature is the use of the multigrid vacuum tube to obtain a low resistance in the charging path of a condenser controlling the gain of the compensating vacuum tube, to enable it toA be'quickly charged during marking signals, and a high resistance in thev discharge path for the condenser during the spacing period, thus reducing the amount of drift, i. e., change in control voltage across the condenser due to the varying length of successive marking signal characters, and the resulting characteristic distortion.

These and other features of the circuits of the invention will be understood from the following detailed description thereof when read in connection with the accompanying drawing in which:

Fig. l shows schematically a receiving circuit for a voice-frequency carrier telegraph system embodying the invention; and

Fig. 2 is a curve illustrating the improved operation characteristic obtained in the circuit of the invention.

The receiving circuit of the invention is shown "in Fig. 1 incorporated in one channel of a voicefrequency carrier telegraph system. A receiving lter or other selective network I is provided in the input of the circuit for selecting from the carrier modulated telegraph signal waves received over the transmission line L the particular frequencies assigned to that channel. The output of the filter l is connected through the gaincontrol potentiometer 2 and input transformer 3 to the input of an ampliiier comprising two pentode amplifying vacuum tubes T1 and T2 coupled in tandem through the resistance-condenser coupling circuit 4, the nrst pentode tube T1 preferably being of the variable-mu type.

The transformer 5 couples the output of the second tube stage T2 of `the amplifier across the input diagonal of the full-wave copper-oxide detector bridge D. An adjustable potentiometer 6 shunted by the by-pass condenser 1 is connected directly across the output diagonal of the detector D.

A third tube T3 also of the pentode vacuum tube type has its control grid-cathode circuit connected across the potentiometer 6. The anodecathode circuit of tube T3 includes the windings of the polar receiving relay RR, so that the current output (approximately milliamperes) of the tube, when signals are applied to its control grid-cathode circuit, will operate the relay.

Plate current is supplied from plate battery 8 to the plate of the tube T1 through the resistance 9 of the interstage coupling circuit 4, to the 'plate of the tube T2 through the primary winding of output transformer 5 and to the plate of the tube Ta through part of the windings of the relay RR.

Suitable heating current is supplied from the filament battery I0 through the series resistance Il to the heaters of the heater type cathodes of the tubes T1, T2 and Ti/in series.

The suppressor grids of the tubes T1 and T2 are connected directly to the cathodes of the respective tubes. The screen grid of the tube T1 is shown connected directly to ground, which is 20 volts above cathode potential, the screengrid of the tube T2 is connected directly to the positive terminal of plate battery 8 and the screen grid of the tube T3 is connected directly to the plate of that tube. The screen grids of the tubes may, of course, be connected to other points of positive potential.

The parallel resistance-condenser grid biasing arrangement I2 is connected in the cathodecontrol grid circuit of the tube T2. The control grid of the tube T3 is connected to the upper terminal I4 of the potentiometer resistance 6 through the large series resistance (10 megohms) I3. The suppressor grid of the tube T3 is connected to the variable tap I5 on the potentiometer 6.

A resistance I6 in parallel with the compensating condenser I'I is connected in the control grid-cathode circuit of the tube Ti between the cathode thereof and the lower terminal of the secondary winding of input transformer 3, so that the negative terminal of condenser I'I is connected to the control grid of tube T1. The lower terminal I8 of the potentiometer 6 is connected to a point between the parallel resistancecondenser network I6, I'I and the secondary Winding of the transformer 3, so that the latter network is connected also in common to the control grid-cathode and the suppressor grid-'cathode circuits of the tube T3, and the negative terminal of condenser II is connected to the control grid and suppressor grid of tube T3.

A xed negative bias is supplied from the filament battery Ill to the suppressor and control grids of tube T3 over paths which may be traced from the negative terminal of that battery through resistances II and IG and the lower portion of potentiometer 6 to the tap I5 thereon, from tap I5 directlyto the suppressor grid, and from tap I5 through the upper portion of potentiometer 6 and resistance I3 in series to the control grid.

'Ihe plate battery 8 supplies biasing current to the windings of the receiving relay RR through the resistances I9, and the by-pass condenser 20 is connected between the plate and the cathode across the windings of the relay.

The armature of the receiving relay RR is connected to a receiving telegraph loop When the armature of the relay RR is on the marking contact M as indicated, the battery Bi is connected to the receiving loop, and when the armature is on the spacing contact S, the battery B2 is connected to the receiving loop, respectively, to actuate sounder or other apparatus to indicate marking and spacing signals in accordance with well-known telegraph practice.

'I'he receiving relay RR is initially adjusted, for example, by proper selection of the values of resistances I9 in the biasing circuit to make the amount of biasing current supplied to its windings from battery 8 such that the relay will operate to and fro in response to the plate current of the controlling tube Ta as nearly at the midpoint on the tube characteristic as isv feasible. Thus, the relay will pick up and fall away at the proper time intervals depending on the length of the applied marking and spacing signals for a given intensity of the received signals.

The circuit of the, invention, as shown in Fig. 1, operates in the following manner to prevent the advance or retarding of the pick-up and fallaway times of the relay, or changes in signal bias, tending to change the marking and spacing signal durations, with changes in level of the applied signals.

'I'he potentiometer 2 iS normally Set to give the proper circuit gain at the time of installation, and generally will not need further adjustment unless the normal received energy level is radically changed. Line-up of the circuit consists merely in transmittingr dots (23 dots per second) through the receiving filter I and adjusting potentiometer 2 until the signals transmitted by the receiving relay RR are unbiased.

The incoming carrier wave modulated with the marking and spacing signals generated at the transmitting end of the system (not shown), received over the transmission line L are impressed on the receiving filters in the receiving channels at the receiving station. That portion within the frequency range of the receiving lter I in the receiving channel, illustrated in detail in Fig. 1, will be passed thereby and with an intensity regulated by the setting of the potentiometer 2 will be impressed by the input transformer 3 on the input of the amplifier and will be amplified by the amplifying tubes T1 and T2 therein in accordance with the initial gain setting of these tubes,

The amplified waves in the output of the second tube T2 of the amplifier will be impressed by transformer 5 on the full wave copper-oxide detector bridge D and will be demodulated thereby. 'I'he resulting unidirectional current, with the carrier components largely filtered out by the condenser (0.05 microfarad) 1, will flow through the potentiometer resistance 6 producing a voltage drop therein which is applied to the control grid-cathode circuit of tube T3 connected across said potentiometer, operating as a direct-current amplifier. The resulting amplified signal current in the plate-cathode circuit of the tube Ta will cause the operation of the receiving relay RR to its marking M contact in response to a received marking signal and to the spacing contact S, in response to a received spacing signal.

Direct-current amplification is used in the last amplifier stage inorder that a large output may be obtained from the high impedance pentode tube Ts. I'his tube is worked at an unusually high negative control grid bias to provide a high degree of signal shaping, thereby making the signal bias more independent of receiving relay variations than would otherwise be the case.

The gain of the receiving circuit is automatically adjusted to compensate for changes in the received carrier level by the portion of the rectified output voltage of the copper-oxide detector D applied from the tap I5 of potentiometer E to the suppressor 'grid of the tube T3. 'I'he resulting grid current flow when the grids of tube T3 become positive, charges the compensator condenser I1 shunted by resistance I6. Due to the effect of the large resistance (10 megohms)y in the control grid circuit of the tube T3, the amount of control grid current flowing through resistance I6 is comparatively small, and the voltage drop in the resistance I5, therefore, will be mainly due to the flow of suppressor grid current of tube Ta therein.

It will be noted that the suppressor grid current of tube T3 flows through the lower portion of potentiometer 6, the resistances I6 and II and the 24-volt filament battery and ground, The current in this path is in such direction as to make the lower end of resistance IB negative, and this negative potential becomes the control grid potential of the variable-mu tube T1. v

The charge applied to condenser I1 being proportional to the voltage drop produced in the potentiometer 6 by the detected signal variations in the output of the detector D, will be a function of the incoming carrier level. The condenserresistance arrangement I1, I6 being common to the control grid-cathode circuits of the tubes T1 and T3, a negative potential varying in accordance with the signal variations will, therefore. be applied to the grids of these tubes and will cause a proportional adjustment of gain therein.

The negative terminal of the condenser I1 being connected to the control 4grid of the tube T1, the gain of that amplier tube will be varied in a direction and to an amount such as to tend to maintain the output voltage of the following detector D constant for an extended range of input variations. A further compensating action is obtained in the last amplifier tube Ta, the control grid of which is also connected to the negative end of the compensating condenser I1.

To minimize the eilects on the bias of signals of changes in the receiving relay RR, it is desirable to supply a large operating current and a steeply rising signal wave front to this relay. A steeply rising signal wave front is obtained from the considerablv rounded carrier signal by biasing the control grid of the tube T3 supplying the relay operating current. well beyond cut-off so that a small percentage change in the signal voltage applied to the grid causes a large change in the plate current of the tube. .The tube 'T3 selected for this purpose was one which requires a rather low value of negative grid bias to reduce the plate current to zero. This made it possible to use the filament battery to supply a grid biasing potential large compared to the cut-off potential thus making it unnecessary to use an auxiliary grid potential supply circuit.

A relatively large relay operating current is obtained from such a small tube by rectifying the carrier in the full-wave copper-oxide detector. suppressing the carrier frequencycomponents of demodulation and impressing the direct-current signal voltage on the grid of the tube T3. In this way, the full value of plate current of the tubey T3, as determined bv the static characteristic of the tube, is obtained. This is an improvement over the Voice-frequency carrier telegraph receiving circuits 'of the prior art in which the carrier is supplied directly to the relay-controlling tube grid, so that plate current flows during only a small part of each carrier cycle. Hence, the current which can be obtained from the tube in the latter case to actuate the relay is considerably less than that which would be ind!- cated by the static characteristic of the tube.

The manner in which the circuit of the invention minimizes drift, i. e.. change in the control voltage across the compensating -condenser I1 due to the varying length of marking signal charactersbeing received. resulting in characterstic distortion, without requiring a compensator relay to prevent discharge of the condenser during a spacing signal as in prior art circuits, may be explained as follows:

The problem may be stated as follows: In a level compensating device in which a voltage proportional to incoming level is established across the resistanceishunted condenser and used to control the gain of the receiving circuit, it is desired that the voltage remain practically ccnstant for a given line equivalent, regardless of the character of the signals being received. 'Ihat is, a voltage produced across the condenser when incoming current is modulated with telegraph signals.

'Ihe explanation will be simpliiied by referring first to the action in an analogous simple circuit in which a battery of voltage E is connected across a condenser C shunted by a resistanceRc, through a resistance Rs and a switch S in` series. With the switch S closed, a voltage ec is established across the condenser C. When the switch is opened, the voltage ec will diminish as Athe condenser discharges through the parallel resistance Rc. If the switch S is' alternately opened and closed for varying lengths of time, as during the transmission of telegraph signals, the condenser discharge occurring during even the longest spacing signal required by the telegraph code can be made small by making the value of the shunting resistance Re'large. Upon the closure of switch S, the lvoltage ec will practically reach a steady state value before the beginning of the next spacing signal, if the series resistance Rs is suciently small. Therefore, by suitably v choosing the constants in this simple circuit, the voltage across the condenser can be maintained at a fairly constant value regardless of the character of the signals being sent by the switch S. Furthermore, if the charging voltage E could be increased and the series resistance decreased momentarily when recharging the condenser after a spacing signal, the condenser voltage could be maintained at an even more nearly constant value.

'I'he application of this basic idea to the carrier telegraph receiving circuit of the invention will now be specically described with reference to Fig. 1. i

The rectified signal voltage output of the copper-oxide detector D is impressed across potentiometer Ii on the control grid-cathode circuit of the pentode tube Ts. A portion of this voltage is applied to the suppressor grid of tube T3 from tap I5 of potentiometer 6. This gives rise to a iiow of grid current from the suppressor and control grids to the cathode. 'I'he control grid current is prevented from becoming very large by the large resistance I3 in series with the 'control grid. 'Ihe suppressor and control grid currents flow through compensator resistance I6 shunted by condenser Il, causing the terminal of that resistance connected to the grid of tube Ts to become negative.

'the slider I5 and-terminal I8 of the potentiometer 6, due to the signal, and the negative potential due to the sum of the fixed biasing voltage Ec rapplied to the control grid of tube T3 and the compensator potential ec across condenser Il. These opposing voltages are relatively large and nearly equal so that their difference is small. That is, the compensator voltage ec is a function of a resultant of two large opposing voltages. A small Vchange in one of the large voltages will produce a large change in the difference voltage. in a direction such as to tend to restore the condition existing before the change.

During the reception of a spacing element of The compensator voltage across the parallel condenser I1 and hence the a telegraph signal, the suppressor grid to cathode resistance of tube T3 being low, the only path for the discharge of condenser l1 is through the parallel resistance I6 which is high in value compared to the resistance in series with the condenser II during the charging period. Because of this, the condenser l1 will discharge slightly thus reducing by a small amount the negative bias on the grid of the variable-mu tube T1 and hence slightly raising the gain of the amplifier.

VThis will result in a small increase in the positive voltage betweenthe slider l and the terminal I8 of the potentiometer 6 when the next marking signal is received. As a result of this small change in the positive and negative potentials, the difference between them will increase by a very large percentage. This gives rise to a very much greater flow of suppressor grid current momentarily. The increase in grid current will be more than proportional to the increase of suppressor grid-cathode voltage because of the non-linear characteristic of the suppressor grid-cathode resistance of tube Ta, which decreases rapidly with increase in voltage. Hence, the condenser charge lost during a spacing signal will be restored very quickly during the following marking signal interval because of the low resistance in the charging path of the condenser Il due to the operated condition of the tube Ts during that interval. In other words, there is only a small amount of drift in the control voltage across the compensating condenser I1 due to the varying length of succeeding marking signal characters, and the characteristic distortion due to this is so small as not to affect appreciably the operating periods of the receiving relay RR.

The small variation in the signal voltage in the output of the copper-oxide detector due to input level change,' not compensated for in the first stage of the amplifier, will be corrected for by the secondary compensating action in tube Ta. This is obtained by applying the voltage of condenser Il not only to the grid of tube T1 but also to the grid of tube T3, and by properly proportioning the values of the iixed grid biasing voltage and compensator voltage.

By selecting the initial grid biasing voltages, such that the change in the compensator voltage, due to the change in signal input level, will be just the correct proportion of the initial grid biasing voltage, the signal current in the output of tube T3 may be made to pass through the value required to operate the receiving relay RR at the proper time intervals to produce undistorted signals.

To illustrate the improvements obtained in the circuit of the invention, reference may be had to Fig. 2 in which changes in input level of the circuit expressed in decibels (db.) are plotted as abscissae and the percentage of telegraph signal bias (at 23 dots per second) is plotted as ordinates. As shown, the percentage of signal bias remains substantially constant over a wide range of input levels extending from zero to 45 decibels.

Although the invention has been described as incorporated in a carrier telegraph receiving circuit, it is apparent that it is capable of use in connection with other types of telegraph signaling circuits.. Also, the gain control circuit employing the suppressor grid current of a pentode tube is adapted for use in` Iother types of signaling systems, for example, in connection with wire or radio telephone systems.

Various other modifications of the circuits illustrated and described Within the spirit and scope of the invention will occur to persons skilled in the art. For example, it is apparent that the potentiometer 2 used for obtaining initial adjustment of the circuit may be connected across the secondary Winding of the input transformer 3 instead of across the primary winding as shown.

What is claimed is:

1. A circuit for compensating for the variations in amplitude level of transmitted signals in a signal transmission circuit, comprising an electron discharge device connected in said circuit, having a cathode, an anode, and a control grid and a second grid spaced consecutively between said cathode and said anode, means positively biasing said anode with respect to said cathode, circuits respectively connecting said control grid and said second grid to said cathode, means to impress said signals on the control grid-cathode circuit, means to apply to said second grid a direct-current voltage varying in accordance with the level of said signals, and means to utilize the variations in the current produced in the second grid cathode circuit when said second grid is driven positive by the applied voltage to control the gain of said transmission circuit in such manner as to produce the desired compensation for signal level variations.

2. The level compensating circuit of claim l, iniyvhich the last-mentioned means comprises a network consisting of capacity and resistance in parallel, connected in common to said control grid-cathode circuit and said second grid-cathode circuit so that the varying voltage produced across said network by the current flowing in said second grid-cathode circuitcontrols the bias on said control grid and thus the gain of said device, and means to reduce the flow of control grid current through said resistance when said control grid is drivenpositive by the impressed signals to a small amount compared to the ow of current therethrough from said second grid.

3. In combination with a signal transmission circuit, means for compensating for variations in the amplitude level of transmitted signals, comprising a variable-mu amplifying electron discharge device having a control electrode, connected in said circuit, a second electron discharge device having electrodes including a cathode, an anode, a control grid and a suppressor grid spaced between said control grid and said anode, circuits respectively connecting said control grid and said suppressor grid to said cathode, said anode being positively biased with respect to said cathode, means applying a fixed negative bias respectively to said control grid and said suppressor grid, a network comprising capacity and resistance in parallel connected in series in the supressor gridcathode circuit of said second device, means to impress said signafs on the control grid-cathode circuit of said second device, means to apply to said suppressor grid a direct current voltage varying in accordance with the level variations of said signals, and means to apply the voltage produced across said network in response to the varying current iiowing in said suppressor grid-cathode circuit when said suppressor grid is driven positive, to said control electrode of said variable-mu amplifying device to proportionately control the gain of the latter device.

4. A carrier telegraph receiving circuit comprising means for amplifying received telegraph waves, means for detecting the telegraph signals therefrom, a telegraph relay controlled by the I detected singals, and means for reducing distortion in said circuit including telegraph bias `with changes in the amplitude level of the received waves, comprising an electron discharge device connected between said 'detecting means and said relay, including a cathode, an anode, a control grid and a suppressor grid, circuits respectively connecting each of the grids to said cathode, means positively biasing said anode with respect to said cathode, means respectively negatively biasing said control grid and said suppressor grid with respect t said cathode, the bias on said control grid being relatively large, a network comprising a parallel condenser and resistance in said suppressor grid-cathode circuit, means for impressing the detected signals on the control gridcathode circuit of said device, means to apply a direct-current voltage varying in accordance with the level variations of the detected signals on said suppressor grid, and means for utilizing the varying voltage produced across said network by the current in said suppressor grid-cathode circuit when said suppressor grid is driven positive with respect to the cathode by the applied direct-current voltage, to control the gain of said amplifying meansin such manner as to compensate for level variations in the detached signals controlling the operation of said relay. l y

5. A carrier telegraph receiving circuit comprising means to amplify received carrier modulated telegraph signals, means to 'detect from the amplified Wave the low-frequency signal components while suppressing the carrier componets, an electron discharge device` having electrodes including a cathode, an anode, and a control grid and a suppressor grid spaced consecutively between said cathode and said anode, and circuits therefor, a telegraph relay` connected in the anode-cathode circuit of said device so that its operation is controlled by the anode current therein, said control grid and said suppressor grid being respectively negatively biased with respect to said cathode, with the control grid bias substantially larger, means to impress the low-frequency signal components of the detected waves on the control grid-cathode circuit of said device, means to utilize a portion of the energy of the low-frequency signal components in the detected waves to apply a direct-current voltage to said suppressor grid varying in accordance with the.

amplitude level of said detected waves, anetwork comprising capacity and resistance` in parallel connected in the suppressor grid-cathode circuit of said device, and means to utilize the varying voltage produced across said network by the current flowing in said suppressor grid-cathode cirtrol grid circuit, said network being connected in said control grid circuit sothat the voltage acrossl said network controls the gain yof lsbaidvariablemu tube.

7. The telegraph receiving circuit of claim in which the detecting means comprises a full-wave copper-oxide rectiiler, the means for impressing the low-frequency signal components of the detected waves on the control grid-cathode circuit of the first electron discharge device comprises a potentiometer resistance shunted by a by-pass condenser, common to said control grid-cathode circuit and the output of said full-wave rectifier, and the means for applying said direct-current voltage to said suppressor grid comprises a connection from that grid to a tap on said potentiometer resistance,

8. The telegraph receiving circuit of claim 5, in which said amplifying means comprises a variable-mu electron discharge device includingV a control grid circuit, said network is also connected in common tothe control grid circuit of said variable-mu tube and the control grid-cathode circuit of the rst discharge device, so that the voltage produced across said network controls the gain telegraph signals from the received waves, a telegraph relay responsive to the detected signals, a network in series with the input of said amplifier, comprising a capacitor and a discharge resistor in parallel, means responsive to the detected signals to charge said capacitor to a potential which is proportional to the amplitude level of 'the detector output to varythe gain of said amplifier so as to compensate for distortion in the signals controlling said relay caused by variations in the amplitude level of said received waves, and means to lreduce variations in the voltage" produced across said network due to the varying length of successive telegraph marking' signals comprising an electron discharge device connected between said detector and said network providing a low resistance in the charging path for said capacitor to enable it to be quickly charged during marking signals, and a large resistance in the discharge path for said capacitor during spacing signal periods.

10. The telegraph receiving circuit of claim 9, in which said electron discharge device is also connected between said detector and said relay and operates as 'a .direct-current amplifier for the detected signals to provide a large relay operating current.

11. The telegraph receiving circuit of claim 9,

in which said electron discharge device comprises a single amplifying vacuum tube having a heated cathode, an anode positively biased with respect to said cathode, a control grid and a second grid electrode spaced between said cathode and anode, both negatively biased with respect toxsaid cathode, and circuits respectively connectingv said cathode to said control grid and said second grid electrode, the output of said detector beingv connected to the control grid-cathode circuit of said tube acrossa potentiometer resistance, the second grid electrode-cathode circuit of said tube including a portion of said potentiometer resistance and said network in series.

JAMES R. 

